CO oxidation on stepped single crystal electrodes: A dynamic Monte Carlo study

We investigate a simple model for the mechanism of the electrooxidation of CO on stepped electrodes by dynamic Monte Carlo simulations. The model is studied in the limits of no and fast CO surface diffusion. In the absence of CO diffusion, the reaction initiates at step sites by adsorption of OH on the steps and reaction with a neighboring CO molecule, resulting in a pre-peak, followed by growth onto the terraces by OH adsorption on the terraces reacting with neighboring terrace adsorbed CO, resulting in a main peak. The presence of these two peaks in both the simulated cyclic voltammetry and chronoamperometry is a direct consequence of the low CO surface mobility, and resembles the experimental voltammetric and chronoamperometric profiles on stepped Rh[n(1 1 1) × (1 1 1)] electrodes [T.H.M. Housmans, J.M. Feliu, M.T.M. Koper, J. Electroanal. Chem. 572 (2004) 79, T.H.M. Housmans, M.T.M. Koper, J. Electroanal. Chem. 575 (2005) 39, T.H.M. Housmans, M.T.M. Koper, Electrochem. Commun. 7 (2005) 581]. The shape and position of the pre-peak are determined largely by a reaction in one dimension at the steps. Initiation of the reaction at empty sites results in the formation of two reaction fronts proceeding to the left and right of the initiation point, leading to a strong initial current growth followed by a gradual decay as multiple “shrinking rows” extinguish one another. After stripping of the step and the first row of CO molecules adsorbed on the terraces, the reaction grows over the terraces by slower adsorption of OH. For high CO surface mobility the simulated voltammograms and transients resemble the experimental results on stepped rhodium surfaces in perchloric acid and stepped Pt electrodes in sulfuric acid.